1. Field of the Invention
The present invention relates in general to an electrically-operated latch, and particularly to an electrically-operated latch for controlling the motion of a linearly movable shaft. The invention is especially adapted for use in automotive brake/transmission safety interlock systems.
2. Description of Prior Developments
U.S. Pat. No. 4,887,702 discloses a brake/transmission safety interlock system for use in automotive vehicles. A manual shift mechanism is connected to a key-operated ignition lock mechanism via a flexible cable. The cable runs through a solenoid that is electrically connected to the vehicle brake system.
A latch device is carried by the cable within the solenoid so that when the solenoid is in a de-energized state the cable is free to move within limits so as to permit manual motion of the shift mechanism out of the "park" position. When the solenoid is in an energized state the cable is prevented from movement in tile direction permitting manual operation of the shift mechanism out of the park position.
The solenoid is electrically connected with the vehicle brake system so that the solenoid is electrically energized only when the brake is actuated, i.e. when the brake pedal is depressed. The solenoid-operated latch therefore serves as an interlock between the brake system and the shift mechanism, such that the manual shifter can be moved out of the park position only when the vehicle is in a braked condition. This promotes vehicle safety.
The solenoid-operated latch disclosed in U.S. Pat. No. 4,887,702 includes a sleeve crimped or otherwise secured to the cable within the solenoid. The sleeve acts as an armature for the solenoid. A tubular pole piece is mounted within the solenoid so that when the solenoid windings are electrically energized the armature sleeve is magnetically drawn toward the tubular pole piece. The magnetic attractive force acts as the latching force to prevent linear motion of the cable in the direction that frees the shifter for movement out of the park position.
When the solenoid windings are de-energized, the armature sleeve is free to move away from the tubular pole piece. The latch device is then in the unlatched condition wherein the cable can be moved to permit manual motion of the shifter out of the park position.
The magnetic latching force is a nonmechanical force that operates axially on the annular end surface of the armature sleeve. The sleeve cross section has to be relatively large to carry the required magnetic flux without exceeding the magnetic saturation limit. The coil has to have a relatively large number of windings, since the latching force is a direct function of the magnetic force.
Another potential drawback with the solenoid-operated latch of U.S. Pat. No. 4,887,702 is that the armature cannot be axially separated from the pole piece by too great a distance, otherwise the magnetic force will be insufficient to attract the armature to the pole piece. Also, the armature sleeve has to be positioned on the cable with some precision, otherwise the armature will not be correctly located in a magnetic sense.
U.S. Pat. No. 5,176,231 discloses an electromagnetic interlock that includes a rod-type shaft having a notch in its side surface. A solenoid is positioned transversely to the shaft so that a linear axial motion of the shaft causes the notch in the shaft to pass through the solenoid axis. The solenoid has a plunger extending from an armature along with solenoid axis in intersecting relation to the shaft axis.
When the solenoid is energized electrically, the plunger is moved into the notch to latch the shaft against linear motion. When the solenoid is de-energized, a coil spring in the solenoid moves the plunger out of the notch thereby freeing the shaft for linear motion.
Movement of the solenoid armature into a bridging position closing the magnetic gap produces a metallic clicking noise that can be disconcerting to some vehicle owners. It would be desirable to have a noise-free, solenoid-operated latch. Moreover, the solenoid has to be precisely located with respect to the notch in the linearly movable shaft. Relatively slight variances in the plunger-notch orientation which occur in normal vehicle production can interfere with proper operation of the latch assembly.